1. Field of the Invention
This invention relates to a semiconductor speech channel switch using PNPN switches as switching elements, which speech channel switch is incorporated in communications equipment such as a telephone exchange system and the like.
2. Description of the Prior Art
In a conventional automatic telephone exchange system, the ringing signal (audible bell) signal), speech current, dial pulses, charging signal and other required signals are applied to necessary equipment through a speech channel switch, for example, a mechanical contact type switch represented generally by a crossbar switch. With the development of an electronic exchange system, however, all the peripheral control circuits provided for the control of the speech channel switch have already been replaced by electronic circuits, and it has been strongly demanded to realize a speech channel switch consisting of electronic elements too for the purposes of further reducing the overall volume of the exchange system, improving the operating performance of the exchange system, reducing the equipment cost, etc.
An attempt was made in the past for realizing a speech channel switch by using gas discharge tubes as the switching elements thereof. With the recent progress of the semiconductor industry, especially progress in integrated circuit techniques, it has become possible to realize a semiconductor speech channel switch. Especially, a PNPN switch is considered to be most suitable for use as the switching element of such a semiconductor speech channel switch.
The PNPN switch has a four layer structure; P, N, P and N layers, with three PN junctions and therefore it has junction capacitances between adjacent layers. Accordingly, if such a PNPN switch is used in a speech channel switch such as a telephone exchange system and the like, there is a drawback of crosstalk due to said junction capacitances. The crosstalk increases as the frequency of the signal treated by the speech channel switch increases. In order to use the PNPN switches as speech channel switches for wide-band communications, therefore, some device to eliminate such crosstalk must be taken into consideration. However, since such a device has been provided by the inventors for other demands, the description thereof is omitted here.
Now, where PNPN switching elements, of the type described above, are arranged in an mXn matrix array to form a speech path switching arrangement, let us consider the manner of selectively driving the arrangement.
As one technique, a driving circuit may be provided for each of the PNPN switches. However, this technique requires that the number of driving circuits correspond to the number of the switching elements of the array so that it is quite complex and costly.
In contrast to this technique, the individual switching elements may be selectively driven by selecting either an incoming line or an outgoing line for signal transmission, and selecting an individual driving circuit for energizing the PNPN switches of the m-th group of switches corresponding to either a column line or a row line of the m.times.n matrix array. This technique requires m driving circuits for the m.times.n matrix array and, accordingly, is economically more advantageous over the above described technique, since the number of driving circuits is reduced. However, this improved technique, from the standpoint of circuit complexity and cost, has its own problem in that crosstalk between the individual PNPN switching element arises.